BQ27Z558: Parameters

Hello Shuji,

Yes you can get a accuracy of 1mA very easily with our system using the same/different sense resistor as seen on the EVM. all the following parameters listed below is founded within this battery gauge expect usage time would be labeled as Total Fw Runtime. I've attached the techincal reference manual that have the address location of the parameter to use/read on bqstuido.

BQ27Z561-R2 and BQ27Z558 Technical Reference Manual:
https://www.ti.com/lit/ug/sluuc54c/sluuc54c.pdf?ts=1743798323841&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FBQ27Z558

Thank you,
Alan

Hi Alan,

Thank you for your reply.

I have an additional question.

On the BQ27Z558 product page, the minimum battery capacity is 100mAh.

The battery capacity used by the customer is 28mAh. Can I use the BQ27Z558?

Best Regards,
Ishiwata

Hello Ishiwata,

Your question has been received and will be answered promptly.

Thank you,
Alan

Hello Ishiwata,

Sorry for the oversight on my end but if your battery doesn't meet the required minimum than you can't use the product.

Thank you,
Alan

Hi Alan,

Thank you for your confirmation.
I understand.

The customer is also considering BQ27426 or BQ27427, but these also have a minimum battery capacity of 100mAh.
Am I correct in understanding that they are not suitable for a battery capacity of 28mAh?

Best Regards,
Ishiwata

Hello Shuji,

Your assumption is correct, 28mAh cell is very small and any kind of minimal discharge will be seen as noise from the fuel gauge end. If they are using a coin cell that is disposable than it doesn't need a BMS of any sort but I would recommend moving up to 100mAh pack either ways. 

Thank you,
Alan

Hi Alan,

Thnank you for your support.

If I were to check the remaining battery charge of a 28mAh battery using the BQ27Z558 or BQ27426, what would be the accuracy?

In a previous answer, you said that an accuracy of 1mA could be achieved, but is it possible to check the remaining battery charge of a 28mAh battery in increments of 1mA?

Also, are there any devices in TI's product lineup that can support the remaining battery charge of a 28mAh battery? If so, please tell me the product name.

Best Regards,
Ishiwata

Hello Shuji,

The lowest cell that we supported was 50mAh that I've head from and after consulting with people on the team, the biggest reason why the battery gauges require 100mAh capacity minimum and why the algorithm does 1mA accuracy is so that the state of charge(SOC) is able to nicely go from 100>>99>>98>>97% and etc. With a 50mAh cell the battery state of charge(SOC) goes from 100>>98>>96>>94% and etc. However the biggest challenge with a 28mAh is not just having to deal with the SOC going from 100>>96>>92>>88% and etc but that your data flash parameters would need to be highly updated to accommodate for the cell along with making a new custom chemID for this cell as well. This undertaking will require a huge amount of support for a low capacity cell and would need to looked over by multiple people/management. Currently in terms of the Texas Instruments battery gauges line up we don't officially support anything below 100mAh.

Thank you,
Alan

Hi Alan,

Thanks for your answer.

Is it possible that there is a difference between the actual remaining charge and the obtained value when the device is at 80% or 0% remaining in the case of a small capacity battery?

Best Regards,
Ishiwata

Hello Shuji,

I want to emphasize that this project would need special approval due the team not having done a battery capacity this small and the likely hood of it being buggy with low levels of accuracy along with unforeseen issues that could come up essentially blocking the device from being uses. I heavily urge your team to reconsider a different solution.  

Thank you,
Alan